Method for operating fuel cell with liquid fuel



June 1955 M. A. WEISS ETAL 3,188,241

METHOD FOR OPERATING FUEL CELL WITH LIQUID FUEL Filed June 21, 1962MALCOLM A. WEISS CHARLES H.woRs|-|A M HUGH H. HOROWITZ 'NVENTORS MANFREDJ. PRAGER BY @ZMB.

PATENT ATTORNEY cathode.

United States Patent 3,188,241 METHOD F02 UlERATlNG FUEL CELL WITHLEQUED FUEL Malcolm A. Weiss, Union, @harles l-li. Worsham, Fanwood, andHugh H. Horowitz and Manfred .l. lrager, Elizabeth, N1, assignors toEsso Research and Engineering Company, a corporation of Delaware FiledJune 21, 1962, Ser. No. 204,135 4 Claims. (Cl. 136--86) This inventionrelates to new and useful improvements in electrochemical cells andparticularly to those cells designed for direct production of electricalenergy from liquid fuels. In particular, this invention relates .to theoperation of electrochemical cells employing an aqueous electrolyte anda liquid carbonaceous reactant that is immiscible with or has limitedsolubility in such electrolyte. More particularly, this inventionrelates to fuel cells employing liquid hydrocarbon fuels floating uponan aqueous electrolyte with the fuel electrode or anode positioned atthe fuel-electrolyte interface.

The term fuel cell is used herein and in the art to denote a device,system or apparatus whereinchemical energy of a fluid combustible fuel,e.g. hydrogen, carbon monoxide, a hydrocarbon or a substitutedhydrocarbon containing hydrogen in its molecular structure, iselectrochemically converted to electrical energy at a nonsacrificial orinert electrode. The true fuel cell is adapted for continuous operationand is supplied with both fuel and oxidant from sources outside the cellproper. Such cells include at least two nonsacrificial or inertelectrodes, functioning as an anode and cathode respectively, which areseparated by an electrolyte which provides ionic conductancetherebetween, conduction means for electrical connection between suchanode and cathode external to such electrolyte, means for admitting afluid fuel into dual contact with the anode and electrolyte and meansfor admitting a fluid oxidant into dual contact with the cathode andelectrolyte. Where necessary or desired, the electrolyte compartment isdivided into an anolyte compartment and a catholyte compartment by anion-permeable partition or ion-exchange membrane. Thus, in each suchcell, a fluid fuel is passed to the anode and there oxidizedelectrochemically, giving up electrons to the anode, while a fluidoxidant is passed to the cathode and there reduced upon receivingelectrons from such Since the voltage developed by an individual cell islow, it is usually preferable to employ relatively small cells and toelectrically connect large numbers of such cells in series or in bothseries and parallel.

The invention is also applicable to electrolytic cells which unlike theaforementioned fuel cells do not provide a net production of electricalenergy but in which an organic fuel is oxidized electrochemically at theanode thereof. In such cells a direct current of electrical energy froman external source, e.g. afuel cell, a storage battery or an alternatingcurrent rectifier, is admitted to the electrical circuit to provide anelectron supply to the cathode. These cells evolve hydrogen from anaqueous electrolyte and water is added to the electrolyte while the cellis in operation. Such cells can be used for electrochemical productionof various organic chemicals, e.g. conversion of alcohols to ketones,hydrocarbons to carboxylic acids, etc.

Cells of the typehereinbefore described-have been successfully operatedwith both gaseous and liquid fuels. Where the fuel is employed as a gas,a porous electrode has proven an effective means for establishing theessential three-way contact of fuel, electrolyte and electrode. Gaseousfuels provide certain problemssuch as pressurized storage, etc. and, ofcourse, many suitable fuels are liquids at the operating temperaturesemployed with aqueous electrolytes.

The problems encountered with liquid fuels which are readily soluble inthe electrolyte are quite different from those encountered withimmiscible fuels.

With liquid fuels which are either essentially immiscible with theelectrolyte or only slightly soluble in such electrolyte theestablishment of the essential three-way contact over a suitablereaction area per unit volume of cell has proven especially diflicult.The feeding of such fuels through a porous electrode had been suggestedbut has not proven as effective as with gaseous fuels. It has also beensuggested to emulsify such fuels with the electrolyte with a view toachieving conditions similar to those occurring with soluble fuels. Thecomplexities of the emulsion art together with problems of diffusion andviscosity control have held back development of a practical applicationof this technique.

It now has been discovered that fuels immiscible with the electrolyte oronly slightly soluble therein can be efficiently oxidized by floatingsuch fuel upon the electrolyte and positioning the fuel electrode oranode at or extending'through the interface. In a preferredembodimentthis combination is used with a small amount of a suitablesurface-active agent added to the electrolyte.

The fuels to which this invention is directed include aliphaticsaturated and unsaturated hydrocarbons such as hexane,.hexene, heptane,heptene, octane and octene, as well as hydrocarbon mixtures such askerosene, gasoline diesel fuel, etc. i The function of the surfactant inthis process. is to increase the electrode area available to the fuelfor electrochemical oxidation without decreasing the over-all'efliciency of' the half cell.

It is surprising that surface-active agents should increase the areaover which reaction occurs near a liquid liquid-solid phase boundary orinterface. Surprisingly, the tests made showthat the reaction areaextends below the electrolyte-immiscible fuel interface, and separateexperiments show that the metal is preferentiallywet by the non-aqueouslayer. Thus, the electrochemical reaction appears to be occurring on anelectrode area covered by a non-conducting medium. So far as it isknown, this phenomenon has not been recognized by the art. Secondly, thedepth of the meniscus, d, is obtained'by the formula d (1-sin 0:)

where 0' is the interfacial tension between the liquids, p is thedifference between the density of the electrolyte and the fuel, g is thegravity constant and a is the contact angle between the nonaqueous layerand the solid surfaces.

In systems observed, or always has been close to zero so that a ISurfactants ordinarily lower the surface tensions of aqueous solutions,i.e. decrease 0. Therefore they should be expected to decrease the areain which reaction occurs rather than increase it.

Furthermore, insertion of the most favorable reasonable values of a andp into theequation last-above stated gives a value for d of less than 4mm; for the useful immersion depth and values for d of less than 2 mm.might reasonably be expected with the employment of a surfactant. To thecontrary, however, it has been observed that the useful depths achievedby aid of the surfactant are considerably larger than where the same isnot employed. This indicates that the reaction is occurring Qconsiderably beyond the calculable meniscus and that the surfactant isenhancing the reaction in this region by some mechanism the exact natureof which is not known.

The fuel electrode employed in accordance with this invention may beeither of the porous variety or impervious plates, both of which arewell known in the art. However, since the area of elfective reactionextends only a limited distance on either side of the interface, thisfact should be considered in the design of a cell for employing afloating fuel. 7

Suitable electrode designs for use with the process of this inventioninclude any of the conventional, porous, nonporous, metallic or carbonand metal structures known to the art for use with aqueous electrolytes.

Electrolytes suitable for use with this invention include both acids andbases among which H 80 H PO NaOI-l, and KOH are typical examples. Acidsare preferred for fuel cell operations in that they permit rejection ofcarbon dioxide produced in the electrochemical oxidation of organicfuels.

In accordance with this invention the surface-active agent employed inthe electrolyte solution can be either nonionic, anionic, cationic or,if desired, mixtures of the same. Of these the nonionic surfactants arepreferred.

The aforementioned general types of surfactants can be furthersubdivided into groups in accordance with their general chemicalcharacteristics. The more common representatives of the groups areincluded in the following list:

The foregoing lists are illustrative but not exhaustive. Other suitablesurfactants are commercially available. Among these are the fluorocarbonsurfactants which may takes the form of one of the aforenamed surfactantgroups, with fluorine substitution for hydrogen atoms in the lipophilicportion of the molecule.

Representative nonionic surfactants for use in this in vention includethose represented by the following type formulas:

wherein R represents an alkyl radical, R a phenylene radical and n and mare positive numbers, the value of n and m and the number of carbonatoms in R providing a balance in conformance with the generalrequirements for the surfactant hereinafter set forth.

When nonionic surfactants are employed it is preferred to employ acompound having an HLB value in the range of about 4 to 8. Solubilitycharacteristics of a surfactant are dependent upon the existing balancebetween the lipophilic portion of the molecule, e.g. hydrocarbonradical, and the hydrophilic portion, e.g. ethylene oxide groups. Ameasure of this balance recognized in the art is the HLB value. Thespecific system of calculating HLB value employed herein is set forth inInterfacial Phenomena, I. T. Davies and E. K. Rideal, Academic Press,111 Fifth Avenue, New York 3, New York,(196l), at page 372, which is tobe considered incorporated herein by reference together with thereferences cited thereon.

Cationic surfactants which may be used include those having an HLB valuein the range of about 0.6 to about 16.3. These are illustrated by thefollowing compounds:

Anionic surfactants which may be used include those having an HLB valuein the range of about 9.4 to 44.5. These are illustrated by thefollowing compounds where n is zero or a positive number in the range ofabout 1 to 15, M is hydrogen ion or other cation, e.g. Na, K, etc., R isan alkyl radical containing about 2 to 12 carbon atoms and R a phenyleneradical.

In general, any surfactant can be used which does not adversely affectthe intended half, cell reaction while extending the effective reactionarea.

The concentration of the surfactant in the'electrolyte solution can varyover a wide range. If the chemical composition of the surfactant is suchthat it competes with the regular organic fuel for electrochemicaloxidation at the anode, it is preferred to operate with lowconcentrations of surfactants which are either periodically orcontinuously replaced. The concentration of surfactant in theelectrolyte solution will ordinarily be above about 0.001 wt. percent,preferably in the range of about 0.001 to 1.0, and most preferably about0.1 to 0.3 wt. percent.

In a preferred embodiment of the invention, the nonionic surfactant is acompound in accordance with the aforelisted type formula RO (CH CH O)I-I wherein the hydrophobic or lipophilic portion of the molecule is analiphatic alcohol having about 8 to 30, preferably about 12 to 18,carbon atoms per molecule. The amount of ethylene oxide employed willvary somewhat in accordance with the number of carbon atoms in thehydrophobic portion of the molecule. Employing tridecyl alcohol as anillustrative example of the hydrophobic groups, the best results areobtained when this is combined with about 12 to 20, preferably about 14to 16, ethylene oxide units per molecule.

Referring now to the accompanying drawings which illustrate two generaltypes of cells in which the instant invention provides advantages, it isto be understood that each figure illustrates only one of manyembodiments of each type of cell.

FIGURE 1 is a schematic side view of a fuel cell adapted to employ aconventional oxygen or air activated cathodic half-cell and a liquidfuel which is immiscible with the electrolyte.

FIGURE 2 is a schematic side view of an externally powered electrolyticcell adapted for chemical production from a liquid, organic fuel whichis immiscible with the electrolyte.

Referring now to FIGURE 1, there is shown a cell container 1 of suitablematerial, e.g. glass, hard rubber, corrosion resistant metal, etc.,having positioned therein an anode 2 and a cathode 3, and anion-permeable or ionexchange cell divider 4 which partitions thecompartment means for the escape of excess oxidant or other gases fromoxygen receiving zone 8. Since the cathodic half cell is not directlyinvolved with the essential novelty of this invention it becomes obviousthat any suitable cathode or oxidant adapted for use with an aqueouselectrolyte can be used. Thus, in lieu of the direct reduction ofgaseous oxygen at the cathode a liquid oxidant such as nitric acid canbe used and the reduction product thereof regenerated with gaseousoxygen or air.

In the anodic half-cell anode 2 is here shown as a metal sheet which maybe coated with a suitable catalyst comprising one or more metals whichhave been deposited thereon either chemically or electrochemically. Thecomposition and structure of anode 2 may also be varied so as to employany of the suitable electrode types known to the art so long as the sameis compatible with the electrolyte system employed.

Catholyte compartment 5 and anolyte compartment 6 each contain anaqueous electrolyte, e.g. 30 wt. percent H 50 5A and 6A, respectively.On top of the anolyte 6A there is floated a layer of fuel 6B which isessentially immisible with the electrolyte, e.g. heptanev Anolyte 6Aalso contains a small amount of a surface-active agent as hereinbeforedescribed. Fuel inlet conduit It? provides means for admitting fuel 63to anolyte 6A. Anode 2 is positioned so as to extend through theinterface formed by 6A and 6B. Anolyte exhaust conduit 11 providesescape means for gases, e.g. CO formed in anolyte zone 6. Anolyte inletconduit 12 provides means for admitting surfactant or fresh electrolyteto anolyte zone 6 and can also be used for draining such compartment,removing an electrolyte-product mixture, etc.

Anode 2 and cathode 3 are electrically connected by wires 13 and 14 andresistance means 15 which is symbolic of any device or apparatus adaptedto use direct electric current for activation, e.g. a light bulb. Thecell is, of course, operative without means 15. Referring now to FIGURE2 there is shown an externally powered cell for use' in chemicalproduction which is encompassed by a cell container 101. Insidecontainer 101 are positioned two sheet-like electrodes, anode 102 andcathode 103. The compartment formed by container 101 is partitioned bydivider 194 into a catholyte zone S and an anolyte zone 1%. A source ofdirect electric current, e.g. a storage battery, a fuel cell pack, or analternating current rectifier is electrically connected to anode 192 viawire 114 and to cathode 193 via wire 113. Inside catholyte compartment105 and anolyte compartment 106 is an aqueous electrolyte, e.g. H 80KOH, etc. 195A and 106A, respectively. Floating on anolyte 1196A is aliquid organic material, e.g. octene which is essentially immisciblewith the aqueous electrolyte. Anode 102 extends through the interfaceformed by 106A and 1063. When power is admitted to the circuit via powersource 115 the organic reactant 106B is electrochemically oxidized atanode Hi2. Such oxidation is controlled by means known to the art so asto remove partial oxidation products of 196B rather than allowing thesame to be converted to carbon dioxide which is the desired product inthe fuel cell operation previously described. At the same time hydrogengas is formed at cathode 103 from the aqueous catholyte 105A and escapesfrom the cell via catholyte exhaust conduit 168. Catholyte inlet conduit16'? provides means for supplying make-up water to the catholyte.Returning now to the anolyte zone, reactant inlet conduit 11 0 providesmeans for admitting the liquid reactant 106B to the anolyte luuA.Anolyte exhaust conduit 111 provides means for the escape of any gaseousmaterials formed by the electrochemical oxidation of 18613 and the loweranolyte conduit 112 may be used to replenish electrolyte 106A or toremove an electrolytepartial oxidation product for separation andrecovery.

The ell dividers 4 and 1954 in FIGURES 1 and 2 are cell component'sknown. to the art and may consist of an ion-exchange membrane, a glassfritor other suitable ion-permeable materials.

The components of the electrodes in all the modifications describedabove are well known and need not be described in detail. Suitablecatalysts, electrolytes, oxidants and fuels for cells employing liquidfuels have been described often in the literature and need not befurther detailed herein since this invention is concerned with a methodof fueling or supplying reactant to electrolytes and to the use ofsurfactants in such electrolytes.

The invention will be more fully understood from the following exampleswhich are for purposes of illustration only'and should not be construedas limitations upon the true scope of the invention as set forth in theclaims.

EXAMPLE 1 v The effect of the addition of surfactants to theelectrochemical oxidation of a liquid fuel floating upon an aqueouselectrolyte was tested by changing the position of the anode, fuelelectrode, and measuring the current generated at different positions.Control runs were made without a surfactant.

A layer of a hydrocarbon fuel, i.e. Z-heptene, was floated upon anaqueous sulfuric acid electrolyte containing 3 moles H 50 per literexcept in the one test indicated. Electrolyte temperature was maintainedat about F. An anode formed of a sheet of platinum foil 1.8 cm. wide andcoated with platinum black was passed through the hydrocarbon phase, theinterface between the hydrocarbon phase and the aqueous phase, and intothe aqueous phase. The cell was operated at varying :depths of immersioninto the aqueous phase to determine the useful area on the surface ofthe anode, i.e. that area Table I EFFECT OF NONIONIC SURFACTANT ON FUELCELL REACTION WITH 2-I'IEPTENE FLOATED ON H2804 ELEC- TROLYTE WITH ANODEEXTENDING THROUGH FUEL- ELECTROLYTE INTERFACE Current Derived at 0.86UseiulAnode Volts Polarization vs. Surfactant Immersion in Standard HReference Acid Layer in mm.

Amps/Ft. A1nps. 10

1 Ethylene oxide condensate with or arlduct of trideoyl alcoholcontaining 9 moles ethylene oxide per mole of tridecyl alcohol.

1 Ethylene oxide condensate with or adduct oi tridecyl alcohol having a30/1 mole ratio of oxide to alcohol.

3 Ethylene oxide condensate with or adduct of tridecyl alcohol having a15/1 mole ratio of oxide to alcohol.

4 Electrolyte contained 0.5 mole H SOJliter.

7 EXAMPLE 2 A cell was operated as in Example 1 with the followingdifferences. The anode employed was a platinum black coated glass frithaving an average diameter of 1.8 cm. No surfactant was added to theelectrolyte. A maximum current of 25 milliamperes was obtained at 3 mm.immersion. The current density at this level of immersion based onuseful electrode surface was 10 miiliamperes/cm.

A further test was made using 0.1 wt. percent of a surfactant, 15 molesethylene oxide adduct of tridecyl alcohol, in the electrolyte. At 1.2cm. anode immersion a current of 75 milliamperes was obtained.

EXAMPLE 3 An electrochemical oxidation of Z-hept'ene was carried outusing a 0.5 molar H 50 electrolyte at 180 F. and an anode comprising aplatinum black coated glass frit having an average diameter of 1.8 cm.The frit was positioned so as to extend through the heptene-electrolyteinterface and various surfactants were tested for effect upon thereaction. The amount of surfactant employed was in all cases 0.1 wt.percent based on total electrolyte solution. The results of these testsare set forth in the following table:

T able II EFFECT OF VARIOUS SURFACTANTS ON OXIDATION OF 2-HEPTENEFLOATED ON H2SO4 ELECTROLYTE WITH Pt COATED GLASS FRIT ANODE EXTENDINGTHROUGH INTERFACE Milliampcrcs at Indicated Surfactant HLB Voltage FromSt. H; Ref. Surfactant Type Value 0.5 i 0.6 l 0.7 i 0.8 I 0.9

E 1 Anionic 36. 7 8. 1 l6. 5 27 43 61 F Cationic 11 4. 8 10.5 24 36 51Nonlonic 5. 7 9. 9 23. 5 50. 7 60 6.7 13.0 27. 5 52 66 7. 2 12. 4 25. 545. 5 76 100 7. 7 9. 7 17. 5 41. 5 72 79 12. 6 0. 1 19 40. 5 58 60 4. 38. 2 17 31 45 51 4. 9 S. 5 20 33 45 62 5. 4 7. 5 18 34 52. 5 59. 5 5. 95. 9 17. 5 42. 5 62 71 ll. 7 10. 9 21. 5 40 46 46 7. 7 9. 2 17 30 37 1 5mole ethylene oxide adduct of tridecyl alcohol sulfate. 2 Lauryldimethylamine oxide.

3 mole ethylene oxide adduct of tridecyl alcohol.

4 12 mole ethylene oxide adduct 0t tridecyl alcohol.

13.5 mole ethylene oxide adduct of tridecyl alcohol. 15 mole ethyleneoxide adduct of tridecyl alcohol.

7 30 mole ethylene oxide adduct of tridecyl alcohol.

6 7.5 mole ethylene oxide adduct of nonyl phenol.

t 9.5 mole ethylene oxide adduct oi nonyl phenol.

10 11.0 mole ethylene oxide adduct of nonyl phenol. 11 12.5 moleethylene oxide adduct of nonyl phenol. 12 30 mole ethylene oxide adductof nonyl phenol.

EXAMPLE 4 A further test was made using a saturated hydrocarbon fuel,n-decane, floating upon a 0.5 molar H 80 electrolyte. The temperaturewas 180 F. and the anode, fuel electrode, was a glass frit upon whichplatinum black had been electrodeposited. The results obtained with andwithout a surfactant are given in the following table:

C) Table III EFFECT OF SURFAOTANTS IN ELECTROCHEMICAL OXI- DATION OFN-DECANE FLOATED ON HzSOi ELECTRO- LYTE WITH Pt COATED GLASS FRIT ANODEEXTEND- ING THROUGH INTERFACE Current in Amps. X 10 at Indicated VoltageFrom St. H1 Ref. Surfactant 1 50/50 mixture of 12 and 15 mole ethyleneoxide adducts of trldecyl alcohol.

EXAMPLE 5 Further tests are conducted as in Example 3 with the samesurfactants except that a concentration of 0.001 wt. percent surfactantis employed in one series of tests, 0.01 wt. percent is employed in asecond series of tests, 0.3 wt. percent in a third series of tests, 0.5wt. percent in a fourth series of tests and 1.0 wt. percent in a fifthseries of tests. An increase in total current density above thatobtained with the same fuel without a surfactant in the electrolyte isobtained in each test.

The term surfactant, the properties and the effects of surface-activeagents are discussed in detail in Textile Chemicals and Auxiliaries WithSpecial Reference to Surfactants, second edition (1957), ReinholdPublishing Corp, New York, New York. See particularly pages 302-3 19.

The surfactant ordinarily will be added directly to the electrolyte butmay be added to the system with the fuel. The requirement of being inthe electrolyte is satisfied by the surfactant reaching thefuel-electrolyte interface.

What is claimed is:

1. In a process of anodically oxidizing a liquid carbonaceous fuel atthe anode of an electrochemical cell while in contact with an aqueouselectrolyte, the improvement which comprises floating said fuel which isimmiscible in said electrolyte upon said electrolyte, positioning saidanode so as to extend through the resulting fuel-electrolyte interfaceand introducing about 0.001 wt. percent to 1.0 wt. percent of an organicnonionic surface-active agent comprising in combination a lipophiliccomponent and a hydrophilic component into said electrolyte.

2. A process in accordance with claim 1 wherein said surface-activeagent is an ethylene oxide adduct of an aliphatic alcohol wherein theratio of the number of moles of ethylene oxide in the hydrophilicportion of the adduct to the number of carbon atoms in the lipophilicportion of the adduct is in the range of about 12 to 20/13.

3. A process in accordance with claim 1 wherein said surface-activeagent is an ethylene oxide aduct of tridecyl alcohol containing about 12to 15 moles of ethylene oxide per mole of alcohol.

4. A process in accordance with claim 1 wherein said surface-activeagent is an ethylene oxide adduct of an alkyl substituted phenolcontaining about 7 to 30 moles ethylene oxide per mole of alkyl phenol.

References Cited by the Examiner UNITED STATES PATENTS 9/45 Gunn et al.136-86 2/60 Justi et al. 13686

1. IN A PROCESS OF ANODICALLY OXIDIZING A LIQUID CARBONACEOUS FUEL ATTHE ANODE OF AN ELECTROCHEMICAL CELL WHILE IN CONTACT WITH AN AQUEOUSELECTROLYTE, THE IMPROVEMENT WHICH COMPRISES FLOATING SID FUEL WHICH ISIMMISCIBLE IN SAID ELECTROLYTE UPON SAID ELECTROLYTE, POSITIONING SAIDANODE SO AS TO EXTEND THROUGH THE RESULTING FUEL-ELECTROLYTE INTERFACEAND INTRODUCING ABOUT 0.001 WT. PERCENT TO 1.0 WT. PERCENT OF AN ORGANICNONIONIC SURFACE-ACTIVE AGENT COMPRISING IN COMBINATION A LIPOPHILICCOMPONENT AND A HYDROPHILIC COMPONENT INTO SAID ELECTROLYTE.